The muscles may still be pumped, but old-school action heroes aren't aging well at the box office.

The crew of The Expendables 3, the latest installment in Sylvester Stallone's blow-everything-up franchise, couldn't muster an explosive opening weekend, bowing to two reigning hits and a fellow newcomer.

Teenage Mutant Ninja Turtles had a second consecutive win at the box office with $28.4 million, according to studio estimates from ticket-sales firm Rentrak.

The heroes in a half-shell rolled past the $100 million mark while also holding off strong competition from Guardians of the Galaxy, which earned $24.7 million. Marvel Studios' superhero space opera is No. 6 on 2014's overall box-office chart with $222 million.

Ninja Turtles is one of the rare films this summer to hold on to No. 1 for two weeks in a row because of its family appeal, says Rentrak senior media analyst Paul Dergarabedian. "This is filling a nice void there and offering up something fun, over the top and action-packed."

The buddy comedy Let's Be Cops, whose humor didn't arrest critics, made its debut in third place with $17.7 million for the weekend, meeting expectations. It has totaled $26.1 million since opening Wednesday, despite an 10% "rotten" critics' rating on aggregate review site Rotten Tomatoes.

Expendables 3 racked up $16.2 million for fourth place, the franchise's worst opening yet, even with a PG-13 rating rather than an R. The first movie had an opening salvo of $34.8 million in 2010, and the 2012 sequel made its debut with $28.6 million.

The novelty of the all-star ensemble cast helped the original Expendables, but though audiences usually flock to sequels, "there is always that potential that if you get to a No. 3, No. 4, No. 5, then audiences start going, 'Well, we've seen this before,' " says Dergarabedian.

However, he adds, "these movies do well overseas and on home video. so there's a long life for action movies beyond the theatrical."

The Jeff Bridges drama The Giver, based on Lois Lowry's popular young-adult novel, rounded out the top five with a $12.8 million debut.

While the summer of 2014 thus far has struggled to meet last year's record-breaking season — it's down 15% to date, according to Dergarabedian — August is up 14% from 2013, thanks to the one-two punch of Turtles and Guardians.